Name ______Date ______
IDENTIFICATION OF SEDIMENTARY ROCKS
Sedimentary rocks form from material which has accumulated on the surface of the Earth under conditions we can readily observe. They are the easiest rocks to understand. We will be looking at 9 representative samples of the major varieties. There are hundreds even thousands of variations on the basic classification theme below.
A.Clastic or Detrital Sedimentary Rocks
These are formed from the broken bits and pieces of previously existing rocks. Quartz is the most common mineral in these sediments because quartz is a hard, tough mineral and is stable chemically at the surface of the Earth. Feldspars, the most common minerals in igneous and metamorphic rocks decompose chemically into clay minerals and dissolved materials. Clay minerals are a major component of shale, the most common sedimentary rock.
Composed of a variety of rounded grains ranging in size from CONGLOMERATE
less than 1/16 mm (0.06 mm) to rocks as big as your head.
Most commonly think of a rock made up ofgravel. Quartz
Pebble is the most common kind of conglomerate.
Similar to the above in grain size only the fragments or "clasts" BRECCIA
are angular. Many breccias have unique and fascinating
stories to tell.
Composed of quartz grains from 2 mm down to 1/16 mmQUARTZ
(0.06 mm). You can test for quartz by scratching glass. SANDSTONE
There are many other varieties of sand stone based on the
mineral composition of the sand composing them.
Composed of clay minerals and clay sized (less than 1/256 mmSHALE
0.004 mm). A true shale is fissile - it splits into thin layers.
You can scratch it easily with a knife or the corner of the
glass plate. Shale can vary in color from black to gray
to red and greenish.
B. Chemical and Biochemical Rocks
These are rocks which resulted primarily from chemical or biochemical precipitation of material from water, usually sea water.
Composed primarily of the mineral calcite which will fizz easilyLIMESTONE
with dilute HCl (hydrochloric acid) can be scratched easily
with the corner of the glass plate. The vast majority of
limestone is the result of biological activity. This latter
fact may only revealed by microscopic study.
Similar to the above only containing abundant fossils. Some-FOSSILIFEROUS
times the rock is entirely composed of fossil fragments. LIMESTONE
Composed of the mineral dolomite this nondescript rock isDOLOSTONE
distinguished by being softer than glass and only the powder
fizzes in HCl. Dolostone is the result of limestone being
"dolomatized" Mg++ + 2CaCO3 = CaMg (CO3) + Ca++.
How this happens is largely a mystery.
Hard (scratches glass), white to gray to black, breaks in large FLINT when black
smooth curved surfaces. Commonly has a soft candle wax luster. or CHERT when white
Composed of microcrystalline quartz, Native Americans used
this rock for arrow heads, scrapers and knives because it could
be flaked into sharp edges.
Composed entirely of the mineral halite. Usually has a peculiarROCK SALT
translucent look. Unfortunately the only sure test is taste.
Yucck! This rock is the result of the evaporation of some large body of
water, usually sea water.
Black! Obvious low specific gravity, soft and soils hands.COAL
The result of terrestrial (on land) accumulation of biological
material. The evidence for the environment of accumulation
usually suggests a swamp.
C. There are many variations on these basic themes.
shaley limestonelimey shalesandy shale
shaley sandstonedolomitic limestonecherty limestone
These are just a few variations based on combining basic forms. There is a world of sedimentary rocks when the simple mineralogy of the rocks discussed is varied.
D. The Exercise:
1. Write the number of the specimens in numerical order.
2. Describe grain size and/or texture
microscopic fine - less than .5 mm ( mm scale on reference chart)
coarse - .5 mm to 2 mmvery coarse - larger than 2 mm
smooth
3. Color/hardnessAs well as color, will it scratch glass or glass scratch it?
4. Mineral composition and other properties.
Minerals detected
Quartz - hardness & lusterHalite - look and taste
Calcite - fizzes in HClDolomite - only powder will fizz
Properties observed
Fizzes in acidSplits into thin layers
Smooth fracture patternFossils present
Tastes salty
5. Name the rock.
E. Structures in Sedimentary Rocks
Explain what the following structures might tell you about the environment when the sediment was being deposited. The structures listed will tell you whether the deposit of sediments was on land or under water. They may also hint of climate and even whether it was fresh water or sea water.
e.g. Mud Cracks-It might have been a tidal flat. When the tide went out, the mud dried in the sun and cracked.
Ripple marks and "fossil"-______
dune structures in a sandstone______
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Fossil corals and sponges-______
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Clam fossils in a sandstone-______
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Other RocksObservations
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